In recent times, singlet oxygen 1O2 has distinguished itself as oxidizing agent of a variety of double-bond systems. Usually, it is obtained by photochemical routes, by dye molecules, such as for example eosine or methylene blue, excited by electromagnetic radiation giving off their excitation energy to oxygen upon reverting to the ground state and consequently converting it from the triplet state to the singlet state. However, this method of producing singlet oxygen requires the use of strong lights, is therefore expensive and not very suitable for use in large chemical plants.
Another way of producing singlet oxygen is pursued by J.-M. Aubry in Tetrahedron Letters 43 (2002) 8731-8734, in which he releases it from an alkaline H2O2 solution by means of a molybdenum compound.
However, this procedure is burdened with a number of disadvantages.
For example, in the presence of excessively large amounts of H2O2, the tetraperoxomolybdate anion is formed from the molybdenum compound used as catalyst and is not able to release singlet oxygen 1O2 from H2O2. Instead, this requires an oxotriperoxomolybdate, but this is only present in solutions in which H2O2 is present in a low concentration. However, such low concentrations are only achieved if, during the reaction, only as much H2O2 is added as disintegrates again through the release of singlet oxygen. Consequently, a very precise dosing of H2O2 during the reaction is required, involving greater expenditure on apparatus, a slower reaction rate and therefore a low space-time yield. Alternatively, there remains only the option of producing singlet oxygen discontinuously, i.e. in a batch process, and tolerating lower singlet oxygen yields and again longer reaction times.
In order to achieve an adequately high reaction rate, according to Aubry, molybdate ions are to be introduced as catalyst in a concentration of 0.1 mol/l, i.e. in a very large amount (see page 8732, column 1, 1). This increases the manufacturing costs of singlet oxygen 1O2 and of secondary products oxidized with it.
Moreover, according to Aubry, the yield of singlet oxygen 1O2 also depends on the type of counterion which is assigned to the reaction-catalyzing molybdate ion. For example, particularly good yields were achieved with lithium as counterion, but not with potassium (see page 8733, 1st column, 2nd paragraph). However, lithium molybdates are very expensive compounds and therefore make the production of singlet oxygen and thus the oxidation of chemical compounds in general, and of mesitol in particular, more expensive.
In addition, molybdenum compounds in the presence of H2O2 are only of limited suitability for the reaction with mesitol since the singlet oxygen 1O2 to be produced catalytically for this purpose oxidizes mesitol not only to the quinol, but also to the perquinol, with both compounds being formed in the ratio 1:1 (see table 2). Before further reaction, this perquinol has to firstly be reduced to the quinol, which leads to additional expenditure.
Finally, Aubry was able to establish that the molybdate-catalyzed disproportionation of H2O2 in the presence of molybdate ions is heavily temperature-dependent and large yields of singlet oxygen 1O2 are obtained at an adequate rate only at elevated temperatures (see page 8733, 2nd column, 2nd paragraph). However, in the case of certain substrates, high temperatures may lead to secondary reactions or else to the thermal decomposition of H2O2.
In the search for further catalysts for the release of singlet oxygen 1O2, Aubry has identified in J. Am. Chem. Soc. (1985), 107, 5844-5849 the alkaline earth metals Ca, Sr and Ba, elements from groups 3a, 4a, 5a and 6a of the Periodic Table of the Elements, actinides and lanthanides, and also ClO−, BrO−, Au+, IO3− and IO4−. Here, he has detected the singlet oxygen 1O2 through reaction with tetrapotassium rubrene-2,3,8,9-tetracarboxylate by oxidizing said rubrene derivative by the generated singlet oxygen in aqueous-alkaline solution to give the corresponding endoperoxide, and determining its content by spectrometry. For various oxidic compounds of the aforementioned elements of the Periodic Table of the Elements listed in table 1 of this article, yields of endoperoxide of 70% were obtained.
Statements about whether the oxidation of the rubrene derivative with the catalyst for singlet oxygen 1O2 given in table 1 proceeds selectively or led to different compound isomers, however, are not made. It is therefore unclear whether the compounds obtained are pure substances or mixtures. In addition, it is also not specified in what amounts and at what temperatures the catalysts are to be used and/or whether larger amounts of H2O2 counteract a rapid formation of singlet oxygen 1O2 and thereby reduce the yield of endoperoxide. Finally, it is not discussed whether the compounds identified in table 1 are also suitable for a specific oxidation of mesitol which, on account of its steric shielding, is significantly less accessible to a chemical reaction.
Consequently, for the person skilled in the art, this gives rise to the task of providing a process, which can be used industrially, for producing singlet oxygen 1O2 and secondary products obtained therefrom which no longer has the shortcomings specified above. In particular, the process should be easy to carry out and be cost-effective, produce singlet oxygen 1O2 in a high yield even at high H2O2 concentrations, and proceed sufficiently rapidly even at room temperature. The provided process should also produce, upon conversion with starting materials, oxidation products as selectively as possible, i.e. with the formation of no or not altogether large amounts of secondary products. Furthermore, it is desired to provide a process with catalysts, the tendency of which to form singlet oxygen 1O2 is only minimally influenced or not influenced at all by counterions. It is also an aim of the invention to arrange individual steps of the inventive process such that a very high yield of singlet oxygen and at the same time a high selectivity with regard to the end product obtained from the prepared singlet oxygen and a starting material is achieved. In particular, it is desired to provide a process with which annular systems such as, inter alia, mesitol can be oxidized regioselectively or largely regioselectively.
Furthermore, the person skilled in the art is presented with the task of providing cost-effective products by means of the process according to the invention, in particular those products which can be used as building blocks for the synthesis of vitamins, in particular of vitamin E.